Differences between carbonates and siliciclasticsSiliciclastic and carbonate sedimentary bodies are subdivided
by similar surfaces that are responses to changes in base level. Differences in the sequence stratigraphy of these sediment
types are related to carbonateaccumulation tending to be "in situ
production" while siliciclastics are transported to their depositional setting.
Rates of carbonate production are greatest close to the air/sea interface since they are linked to photosynthesis and so depth-dependent. Thus carbonate facies and their fabrics are clear indicators of sea level position. Carbonate organisms can produce and accumulate
above certain hydrodynamic thresholds, an effect influenced by their biology
and the chemistry of the water known as ecological accommodation (Pomar (2001 a,
and b). Whereas siliciclastics, which only respond to hydrodynamic thresholds, are limited by their physical accommodation. Thus the character of carbonate sediment changes as organisms evolve, the plate tectonic
configuration of the depositional setting of the basin responds to pale-climate
change, and/or changes in paleogeography related to isolation or access to the
open sea. This means that carbonates can be used as indicators of depositional
setting that, when combined with sequence stratigraphy, make carbonate facies
analysis a powerful tool for the interpretation of the geological section and
lithofacies prediction away from data rich areas.

It should be emphasized that, as has been shown by Fischer (1964), Pomar and Ward (1999), Goldhammer, et al, (1990), and D'Argenio et al (1997) that though shallow cycles of carbonate are composed of a relatively conformable succession of genetically related beds or bedsets these cycles are often truncated and incomplete so that maximum flooding and trangressive surfaces can be missing. This means that these cycles are not, in the strictest sense, a match for the clastic models of parasequences of Van Wagoner et al, (1999). Never the less we argue cycles can be used like parasequences in the analysis of the sedimentary section as units of process/product oriented depositional models. However should they exhibit truncated cycles and miss the sediments of an initial transgression or maximum flooding event one should consider them as high frequency carbonatecycles, not parasequences.

The basic reefal accretional unit of the Miocene reef complex of Mallorca is the "sigmoid ". This is bounded by clear erosion surfaces (the product of sea level lowering and erosion with a matching correlative surface downdip) but has no obvious marine flooding surfaces . Updip and landward the sigmoid is represented by a horizontal lagoonal bed that basinward passes in sigmoidal bedded reef-core lithofacies belt and seaward into clinoform bedded forereef slope beds and sub-horizontal basinal lithofacies. The boundary over the lagoonal and reef-core lithofacies of the sigmoid is formed by an erosional surface that basinward becomes a correlative conformable surface in the reef slope and basinlithofacies. Notably, the coral-morphology zonation within the reef-core facies of the sigmoid migrates seaward, aggrades vertically, or moves landward over the bounding erosional surfaces. This enables the sigmoid (like system's tracts) to be tied to specific segments of the sea-level curve. Consequently the sigmoid configuration can be considered "genetically" as a depositional sequence, though not exactly fitting the original definition of a parasequence. This is because the sigmoid, like the parasequence , is composed of a relatively conformable succession of genetically related beds or bedsets . Also the geometric patterns shown by stacked sigmoids can be used, along with their position within a sequence, like the patterns of stacked parasequence sets, to define system-tracts , while within lower order depositional sequences there are sigmoid sets, sigmoid cosets and megasets.

In the interests of keeping the sequence stratigraphic literature from becoming over complex it is argued here that during the time interval between the development of the erosional surface on the underlying sigmoid and the deposition of sediment marking the boundary of the overlying sigmoid, sea level dropped to be followed by a trangressive flooding event and the development of a maximum flooding surface. However since no sedimentary fill has been recognized that records these events, the sigmoid cannot be inferred to be equivalent to the parasequence, or vice versa! Similarly this "simplification" should not be applied to a shoaling upward carbonatecycle missing transgressive or maximum flooding sediments. In this case the transgressionsurfaces (TS) and maximum flooding surfaces (mfs) are not equivalent to erosion surfaces initially produced by a sea level fall, since the missing sediments mean that one cannot establish how the erosion surface was modified on the following transgression. Clearly the truncated high frequency carbonatecycle may have different genetic elements to a parasequence and should not be considered to be one! It should be noted that because "modern" type of reefal systems are able to build rigid frameworks, resistant to wave energy, this depositional system has the capacity to record even the highest-frequency sea-level cycles. Thus some sigmoids appear to record 7th order sea-level cycles that represent a periodicity of few-thousand years! Other depositional systems that have not produced this "rigid framework" to the sea level are not able to record such high-frequency cycles of sea level and parasequences may form.

Pomar (personal communication, 2004) proposes that parasequences form in response to sea-level para-cycles (rise and stillstand of sea level), commonly as a response of sea-level cyclicity when subsidence equals or exceeds the amount of sea-level fall, OR when the sedimentary systems are dominated by loose grains. In this latter case, lowering of base level (related to the fall in sea level) would increase basinward shedding of sediment and these erosional processes onto a granular seabed would not be recorded as an erosion surface. This could be the reason that higher-frequency sequences (simple sequences in Vail's definition) at the most commonly record up to 5th-order cycles of sea level. These high frequency carbonatecycles that have the genetic elements of the parasequence are "of course" carbonateparasequences.

Some carbonateparasequence geometries - Tools for the interpretation of depositional settingThe sequence stratigraphy of the carbonate sections is commonly determined from a combination of 2 and 3 D Seismic data (providing a comparatively low frequency resolution), well logs (providing a comparatively high frequency resolution), cores (providing very high frequency resolution) and outcrops (with best access to a combination of high frequency resolution and low frequency resolution).

All are the combined products of base level change. This is particularly true of shallow water carbonateaccumulations which are depth dependent, a response to the paleo-oceanography, and processes of the depositional setting. The result of such an analysis creates a "powerful" framework of parasequence and high frequency cycle geometries that can be used to explain, assess and predict reservoir and aquifer quality better independent of thickness and time.

This approach even applies in deepwater settings. For instance, using the Tamabra formation of the Poza Rica Field Area of Mexico as an example, Loucks, et al (2006) have demonstrated that deeperwater mass-transport carbonate deposits are carried by gravity flow and suspension processes into deepwater basinal settings downslope from margins tied to shallow-water carbonate platforms. So while reefal and grain-rich debris accumulate on the shallow platform carbonate debris wedges extend into the deeperwater basin.

The architecture of this debris wedge is related to the availability of source material during changes in relative sea-level (Loucks, et al., 2006). During sea-level lowstands and transgressions or during early highstands when the platform rapidly aggrades, debris and mud flows composed of platform and slope carbonate mud, sand, and clasts generally accumulate. In contrast during highstands of sea level when the platform is flooded and shedding, density-flow and turbidite deposits composed of carbonate sand and lesser amounts of lime mud collect.

Click thumbnail to access the large images and click on the larger image to see them full size!

Geometries of carbonatestrata
The geometries of carbonatestrata are products of the shape of the depositional surface , changing base level and sediment accumulation. They are defined by the underlying and overlying surfaces. These surfaces may be the products of deposition and/or erosion and can coincide with the depositional event or proceed or follow this. Physical erosion, burrowing, boring, dissolution (Clari et al, 1995; Lukasik & James, 2003), and/or cementation may have modified them. Whatever their origin, these surfaces provide a convenient means to subdivide the carbonate section. From the perspective of sequence stratigraphy these surfaces are used to determine the order in which strata are laid down and define the geometries that they enclose.

As with the products of other sedimentary depositional systems carbonatestrata exhibit a hierarchy of scales that include at the small-scale end ( beds , bed sets , and bed cosets) and at the larger spatial scales reef complexes, basin margin and slope complexes etc. These strata can be expressed as unconfined sheets, unconfined but localized build ups (reefs, banks and islands), unconfined but localized sigmoids (reef cores of Pomar 1991), bank margins etc., and confined incised channels (tidal channels and the products of flood events). What ever the final geometry this is the product of both accumulation (aggradation ) and erosion.

A set of carbonate sequence stratigraphy exercises
Click on the highlighted title above to access the exercises that are available on this site to examine the hierarchy of scales expressed by carbonatestrata. These may be the lower frequency subdivisions that can be interpreted from seismic, or higher frequency subdivisions outcrop and well logs. These consider facies or more complex lithofacies assemblages from the perspective of sequence-stratigraphic concepts, including systems tracts, parasequences, sequences and their response to seal level rise ( TST), still stand (HST) fall (FSST) and lowstand (LST) and their response to the paleo-oceanography and processes of the depositional setting. The exercises are intended to develop skills that can be used to establish direct relationships between the nature of the carbonate bodies, the sequence stratigraphic architecture, reservoir connectivity, reservoir characterization and prediction. This would involve the use of systematic hierarchical relationships, integration of seismic, well, and core data with outcrop and subsurface analogs. From this you will gain a better understanding of how to predict accurate net-to-gross, continuity, architecture, and reservoir extent. If you are able to integrate biostratigraphy with your studies this will provide an independent time framework correlation made to cycles of base level rise and fall.

To conclude, carbonate depositional facies hierarchy provides a framework for the systematic description and comparison of carbonate deposits that is based on the physical relationships of strata and their boundaries. The recognition of genetically related stratigraphic elements, is independent of the lithofacies assemblage of carbonate, and is applicable at all scales.